Selectively textured magnetic recording media

ABSTRACT

Magnetic recording media are controllably textured, particularly over areas designated for contact with data transducing heads. In connection with rigid media, the process includes polishing an aluminum nickel-phosphorous substrate to a specular finish, then rotating the disc while directing pulsed laser energy over a limited portion of the radius, thus forming an annular head contact band while leaving the remainder of the surface specular. The band is formed of multiple individual laser spots, each with a center depression surrounded by a substantially circular raised rim. The depth of the depressions and height of the rims are controlled primarily by laser power and firing pulse duration. The shape of individual laser spots can be altered by varying the laser beam inclination relative to the disc surface. On a larger scale, the frequency of firing the laser in combination with disc rotational speed controls the pattern or arrangement of laser spots. The smooth, rounded contours of the depressions and surrounding rims, as compared to the acicular character of mechanically textured surfaces, is a primary factor contributing to substantially increased durability of laser textured media.

BACKGROUND OF THE INVENTION

The present invention relates to the recording, storage and reading ofmagnetic data, and more particularly to rotatable magnetic discs used incooperation with magnetic transducing heads and having at least portionsof their data recording surfaces textured for contact with thetransducing heads.

Magnetic discs and disc drives are well known for their utility instoring data in magnetizable form. They typically employ one or morediscs rotated on central axis, in combination with data transducingheads positioned at close proximity to the recording surfaces of thediscs and moved generally radially with respect to the discs. Generallythese devices are of two kinds. The first uses flexible or "floppy"discs, with associated transducing heads contacting the recordingsurfaces at all times. The second type employs rigid discs rotated atmuch higher speeds than flexible discs. The transducing heads, duringreading and recording operations, are maintained at a controlleddistance from the recording surface, supported on a "bearing" of air asthe disc rotates. The transducing heads contact their associatedrecording surfaces whenever the discs are stationary, when theyaccelerate from a stop, and during deceleration just before coming to acomplete stop.

While all such magnetic recording devices experience at least some wear,the problem is particularly serious for the rigid discs and associatedheads. This is due in part to the stricter design tolerances associatedwith rigid discs and heads, arising from the ever-present challenge ofincreasing the density of data stored on disc recording surfaces. It isconsidered desirable during reading and recording operations to maintaineach transducing head as close to its associated recording surface aspossible, i.e. to minimize the "flying" height of the head. A smooth,specular recording surface is thus preferred, as well as a smoothopposing surface of the associated transducing head. This permits closerproximity of the head to the disc, and more predictable and consistentbehavior of the air bearing supporting the head.

However, if the head surface and recording surface are too flat, theprecision match of these surfaces give rise to excessive stiction andfriction during the start up and stopping of the disc, causing wear tothe head and recording surface which eventually can lead to a headcrash.

In recognition of this difficulty, the recording surfaces of magneticdiscs often are intentionally roughened to reduce the head/discfriction. In particular, rigid disc can be formed with an aluminumsubstrate polished flat and plated with a nickel-phosphorous alloy. Thealloy is polished to a substantially specular finish, e.g. to aroughness of less than 0.1 microinch. The disc is then rotated betweenopposed pressure pads or rollers which support a cloth or paper coatedwith silicon carbide (SiC) or other suitable grit of a sizepredetermined to yield roughness peaks of about one microinch. Peaksthus created tend to be jagged and have sharp edges, and are difficultto control in size, form and location as these factors depend largelyupon the nature of the grit and the direction which the disc movesrelative to the pressure pads or rollers.

In U.S. Pat. No. 4,698,251 (Fukuda et al), a polishing paper is appliedto magnetic discs to form circumferential scratch marks having depthsfrom 0.0002 to 0.1 microns into the nickel-phosphorous alloy layercoated onto an aluminum substrate. Following polishing, a chromium layerand a cobalt nickel magnetic layer are formed on the disc. In anotherembodiment, a nickel-phosphorous layer is deposited onto an aluminumsubstrate, scratches are formed in the nickel-phosphorous layer, then acobalt-phosphorous magnetic layer is deposited on the nickel-phosphorouslayer.

As an alternative to using grit paper or cloth, U.S. Pat. No. 4,326,229(Yanagisawa) discloses a protective film layer for covering a smoothrecording medium layer of a magnetic disc. To form the protectivecoating, a solvent is applied in a spin coating process to form a filmwith radially extending sinusoidal jogs or undulations to increasesurface roughness, which is said to reduce head wear.

While each of the above approaches can be satisfactory under certaincircumstances, all require compromise between the competing goals ofreduced head/disc friction and minimum transducer flying height, andnone affords the desired amount of control over surface texture.

Therefore, it is an object of the present invention to provide amagnetic recording medium in which peaks and indentations formingsurface roughness are of a controlled size and shape for substantiallyreduced flying height, improved recording density, and improvedtransducing head and disc wear characteristics.

Another object of the invention is to provide a process for controllingthe texture of a strictly delineated portion of a magnetic discrecording surface while providing a specular finish on the remainder ofthe recording surface.

Another object is to provide a process for controllably forming thesurface texture of a magnetic data recording disc through control of thesize and spacing between generally circular individual discontinuitiesproviding texture.

Yet another object is to provide a magnetic recording disc withdesignated surface areas for contact with data transducing heads, whichdesignated areas exhibit substantially enhanced friction and wearcharacteristics.

SUMMARY OF THE INVENTION

To achieve these and other objects, there is provided a process formanufacturing magnetic media operated in conjunction with magnetictransducing heads for the recording and reading of magnetic data. Theprocess includes the steps of:

forming a specular substrate surface on a non-magnetizable substratebody, the substrate surface being substantially planar and having anominal roughness;

depositing a magnetizable film over the substrate surface as a layersubstantially uniform in thickness; and

concentrating energy selectively upon a plurality of locations over adesignated treatment area on either the substrate surface or therecording surface, to controllably alter the topography of the selectedsurface at each of the locations, spacing the adjacent locations apartfrom one another a distance substantially less than the length and widthdimensions of a magnetic transducing head positioned proximate themagnetic film for recording and reading magnetic data, the roughness ofthe selected surface throughout the designated treatment area being atleast twice the nominal roughness.

Preferably the concentrated energy is provided by a pulsed or timedlaser which forms at each location a rounded center depression extendinginwardly of a nominal surface plane of the selected surface, and agenerally circular and rounded rim surrounding the center depression anextending outwardly of the nominal surface plane. By rotating the discat a controlled rate corresponding to the pulsed laser frequency, a ringcomprised of a series of depressions and rims is formed, with repeatedrings combining to form an annular band. Preferably, the band is locatedradially inward of a data reading and recording area of the magnetizablefilm. This approach is particularly advantageous in connection withrigid discs and flying heads, in which the inner band is designated as ahead contact area and the remaining surface area is always separatedfrom the head by a bearing of air.

The substrate can include a rigid aluminum disc plated or otherwisecoated with a layer of a nickel-phosphorous alloy, in which case thelaser treated locations preferably are formed in the alloy layer butalternatively can be formed in the aluminum. As further alternatives,however, the laser marks including depressions and rims can be formed inany succeeding layer, e.g. a chromium underlayer, or a cobalt chromiumor cobalt nickel magnetic recording layer. When the laser marks areformed in the aluminum or nickel-phosphorous alloy layers, succeedinglayers tend to replicate them.

Laser surface texturing provides a degree of control previouslyunavailable in grit cloth or paper texturing. The accuracy of the laserenables a precise delineation of the textured area boundaries. The laserpower, pulse length and focusing are variable to control the size andprofile of laser spots or marks. Finally, the pulse frequency, inconjunction with the rotation or other relative translation of the disc,can be controlled to determine the spacing among adjacent marks. Therotund or rounded nature of the laser marks improves the degree ofcontrol in the topography of the magnetic recording media, todramatically improve wear characteristics, in particular as measured byCSS (contact start-stop) testing in conjunction with measuring thecoefficient of friction and self-excited head/gimbal vibration energy.The high degree of control of the pulsed laser produced depressions andrims allows the flying height of the transducing head to be reduced,even over the designated treatment area. Restricting the treatment areaonly to a specific landing zone allows the recording areas to beextremely smooth, thus to allow further reduction in head flying heightin the recording areas, resulting in improved recording density.

Further in accordance with the present invention, the process can beapplied to a flexible magnetic disc, tape or other medium in which therecording surface is generally in contact with the transducing head atall times, and thus the designated treatment area is substantially allof the recording surface. While controlled texturing of flexible discsis not considered as critical as it is with rigid discs, the roundedmarks nonetheless can provide an overall surface texture with reducedfriction and improved head and disc wear characteristics. In a contactrecording system such as this, the controlled, rounded marks can be usedto control head/disc spacing while reducing friction and wear. Inconnection with rigid as well as flexible discs, the generally circulardepressions and surrounding rims are believed to further reducefrictional wear by acting as areas of collection for debris and anylubricant coated onto the disc.

IN THE DRAWINGS

For a further understanding of the above and other features andadvantages, reference is made to the following detailed description anddrawings, in which:

FIG. 1 is a plan view of a rotatable rigid magnetic 30 recording discand a transducing head supported generally for movement radially of thedisc, wherein the disc recording surface includes a designatedtransducing head contact area formed in accordance with the presentinvention;

FIG. 2 is an enlarged partial sectional view of the magnetic disc ofFIG. 1;

FIG. 3 is a schematic view of an apparatus for controllably texturingthe disc in FIG. 1 to provide the head contact area;

FIG. 4 is a substantially enlarged perspective view of a laser markformed in the surface;

FIG. 5 is a schematic view of the profile of the laser mark;

FIG. 6 is a perspective view similar to that of FIG. 4, showing part ofthe transducing head contact area;

FIG. 7 is a perspective view similar to that in FIG. 6 showing analternative surface texture;

FIG. 8 is a schematic view showing a profile of a mechanically texturedsurface; and

FIGS. 9 and 10 are charts illustrating comparative coefficients offriction and self-excited head/gimbal energy, respectively, for surfacestextured in accordance with the present invention as compared toconventionally textured surfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, there is shown in FIGS. 1 and 2 a datarecording medium, namely a rigid magnetic disc 16 rotatable about avertical axis and having a substantially planar and horizontal uppersurface 18. A transducing head support arm 20 is part of a carriageassembly (not shown) supported for linear reciprocation radially of disc16. A magnetic transducing head or slider 22 is supported by arm 20through a head suspension 24, for movement relative to the disc alongwith the arm. Suspension 24 does not support the head rigidly relativeto the arm, but rather allows for gimballing action of the head, i.e.limited vertical travel and limited rotation about pitch and roll axes.

At the center of disc 16 is an opening 26 to accommodate a verticalspindle of a disc drive, not shown, used to rotate the disc. Betweenopening 26 and an outer circumferential edge 28 of the disc, uppersurface 18 is divided into three annular sectors: a radially inwardsector 30 used principally for clamping the disc with respect to thespindle, a head contact region or area or band 32, and a data storageregion or area 34.

Whenever disc 16 is at rest, or rotating but at a speed substantiallybelow its normal operating range, head 22 is in contact with uppersurface 18. But when the disc rotates at least near its operating range,an "air bearing" is formed by air flowing between head 22 and uppersurface 18 in the direction of disc rotation, which supports the head inparallel, spaced apart relation to the recording surface. Typically, thedistance between a planar bottom surface 36 of head 22 and recordingsurface 18, sometimes referred to as the "flying height" of the head, isabout ten microinches or less. Preferably, the flying height is low, toposition head 22 as close to recording surface 18 as possible. Thecloser the transducing head, the more data that can be stored on disc16.

As mentioned above, arm 20 moves to selectively position head 22 overthe recording surface. In this connection it is to be appreciated that arotary arm, although moving head 22 in an arcuate path, could be used inlieu of arm 20 to accomplish substantially the same result. The radialposition of head 22 is controlled before and after reading and recordingoperations, as well as during such operations. More particularly, duringsuch operations head 22 while supported on the air bearing isselectively positioned radially across data storage area 34 to eitherrecord or retrieve data at a particular location on the disc. After suchoperations and during deceleration of disc 16, arm 20 is moved radiallyinward to position head 22 directly over head contact area 32. Thus, bythe time disc 16 decelerates sufficiently to permit the head to engagethe upper surface, head 22 already is aligned with the head contactarea. Prior to the next recording or retrieval operation, accelerationof disc 16 from stop occurs with head 22 initially engaged with area 32.Arm 20 is not actuated to remove the head radially from area 32 untilthe head is supported by an air bearing, i.e. free of the disc.

Because upper surface 18 of the disc includes designated data storageand head contact areas, the surface contours or texture of each area canbe formed in accordance with its function. More particularly, datastorage area 34 preferably is polished or otherwise finished to a highlysmooth, specular finish, having a surface roughness of at most 0.1microinch, to permit the desired low flying height for head 22. Afurther advantage of the specular finish is that, as compared to thetextured surface, foreign particles are readily observed, which enhancesoptical inspection of disc 16. Roughness in this context means theheight of the highest peaks above a nominal horizontal plane of thesurface.

By contrast, head contact area 32 has an a roughness of at least 0.5microinches. The increased surface roughness of the head contact sectorin relation to the remainder of upper surface 18 is achieved by acontrolled texturing of the disc during its manufacture. As seen fromFIG. 2, disc 16 is formed of a multiplicity of layers including asubstrate, a recording layer and a protective cover layer over therecording layer.

More particularly, disc 16 is formed first by polishing, grinding orotherwise machining an aluminum substrate disc 38 to provide asubstantially flat upper surface of the substrate. Next, anickel-phosphorous (Ni-P) alloy 40 is plated onto the upper surface ofthe aluminum disc, preferably evenly to provide a substrate layersubstantially uniform in thickness, e.g. about 10 microns Followingplating, alloy layer 40 is polished to a roughness of generally lessthan 0.1 microinch. For example by a silicon carbide grit lappingprocess. This normally involves a cloth or paper carrying the grit, andcan also involve a liquid slurry containing grit in combination with acloth or paper if desired. Such processes are known and not furtherdiscussed herein.

The preferred stage for the texturing operation is immediately afterpolishing and cleaning alloy layer 40. Texturizing is accomplished withan apparatus illustrated schematically in FIG. 3, including a spindle 42for supporting disc 16, and a pulsed mode Nd-YAG (yttrium aluminumgarnet) laser 44 supported above the disc and generating a pulsed laserbeam 46 selectively focused on the upper surface of Ni-P layer 40. Laser44 is an ESI model 44 laser trimming system available from ESI, Inc. ofPortland, Oreg. Laser 44 is fired at a selected frequency onto the discwhile spindle 42 is rotated, thereby rotating the disc a well. Shown ina vertical orientation, laser 44 preferably is supported by structure(not shown) which permits a tilting of the laser away from the verticalif desired. The support structure further enables a controlled steppedmovement of laser 44 radially of the disc and spindle. The specificapparatus for rotating the spindle and for supporting, orienting andstepping the laser is not shown or discussed in detail as such equipmentis known in the art and not directly concerned with the invention.Rather, the invention lies in the manner in which such equipment is usedto provide a controlled, strictly delineated head contact area 32.

A salient feature of the present invention is the consistent, uniformtexture over the entire head contact area. In achieving uniformity, twolevels of control are involved: a micro level concerned with individuallaser marks or spots, and a macro level concerning the pattern orarrangement of multiple laser spots. The nature of the individual laserspots is controlled primarily by the intensity or peak energy at whichlaser 44 is fired, and the duration of each firing, i.e. the pulsewidth. Somewhat secondary added factors include the way in which beam 46is focused, and the angle of approach. The vertical direction of thebeam upon horizontal substrate surface 48 as shown in FIG. 3, i.e. anapproach angle of 90 degrees, yields substantially circular spots, whilean inclined angle, i.e. 45 degrees, would yield somewhat elliptical oroblong spots.

As for the pattern or arrangement of spots, the primary control factorsinclude the frequency of repeated firings of laser 44, the speed of discrotation, and the amount of radial stepping of the laser.

One preferred texturing approach is to orient laser 44 vertically asshown in FIG. 3, and to maintain it stationary while rotating disc 16and firing the laser at a selected frequency, coordinated with the discrotational velocity to provide a selected distance between consecutivespots. A single rotation of the disc results in a ring of such spotsconcentric on central opening 26. Next, laser 44 is displaced radiallyby a desired inter-ring pitch, and with its vertical orientationmaintained, fired at the predetermined frequency and phase relative todisc rotation. These steps are repeated until a plurality of concentricrings of laser spots form head contact area 32 as an annular band of awidth equal to the pitch times the number of rings.

With the nickel-phosphorous alloy layer thus selectively texturized, theremaining layers, illustrated in FIG. 2, are applied, preferably byvacuum deposition, to complete disc 16. More particularly, a layer ofchrome at a thickness of about 1,000 angstroms is sputter deposited ontothe upper surface of the nickel-phosphorous alloy, to provide anunderlayer 50 for the recording layer. A recording layer 52, which canbe a cobalt nickel alloy, a cobalt chromium alloy or the like, issputtered onto the chromium layer to a thickness of about 500 to 700angstroms. Finally a protective layer 54, for example carbon, isdeposited onto the recording layer at a thickness of about 300angstroms.

As noted above, process parameters primarily controlling individuallaser spots are the peak energy or intensity, and pulse duration, withthe angle of approach and focus also contributing to the size and shapeof any discontinuity. A typical spot formed by laser 44 when verticallyoriented is shown in FIG. 4 and represented in profile in FIG. 5 as asingle laser crater or spot 56. Crater 56 is a combination of twodepartures from a specular surface plane 58 (FIG. 5) which can beconsidered the nominal plane of substrate surface 48, in this case theupper surface of nickel-phosphorous alloy layer 40. The first of theseis a center depression or pit 60, with the other being a substantiallycircular rim or ridge 62 surrounding the pit. The height of rim 62 abovenominal surface plane 58, h in FIG. 5, is preferably in the range offrom 0.5 to 0. 8 microinches, although a head contact area in which rimheights slightly exceed a microinch still can perform satisfactorily.The depth of pit 60 below plane 58, d in FIG. 5, is typically abouttwice the rim height. Thus the surface roughness h of head contact area32 is within a range of from 0.5 to 1.0 microinches. Finally, thediameter of spot 56, which is equivalent to the diameter of rim 62 andrepresented as D in FIG. 5, usually is in the range of 0.1 to 4.0 mils.

The process parameters mentioned above can be varied to influencedimensions D, d and h. The rim height h is considered the most critical,and varies with peak power over a preferred range from about 0.1kilowatts to about 5 kilowatts. The optimum peak power can of coursevary with the particular laser employed, as well as the nature of thesurface being textured. However, in connection with nickel- phosphorouslayer 40, it has been found advantageous to operate toward the low endof the 0.1-5 kilowatt range, just above a point at which melting occurs.

Depth d of pit 60, while not as critical as the height h of rim 62,nonetheless serves a useful purpose, namely the entrapment andcollection of media fragments, head fragments or other debris generateddue to head/disc contact. Further, in connection with fluorocarbonlubricant coatings with tendencies toward liquid behavior, the centerdepressions retain the lubricant coating when the disc is stopped.However, it is believed that as the disc begins spinning, the lubricanttends to travel upwardly out of the depressions and cover the rims, thusreducing dynamic friction in the head contact area.

Another useful feature of the invention is the rounded nature of thecontours forming the pit and rim, with the rounded rims in particularcontributing to substantially enhanced friction and wear characteristicsover the long term. The rounded contours are believed to result from theflow of material due to surface tension forces while the material iscooling, while returning to the solid from the liquid state. When beam46 is fired upon the upper surface of nickel-phosphorous layer 40, thealloy absorbs heat which initially is concentrated in what later becomesthe pit. Heat is rapidly dissipated in all directions radially away fromthe center, principally by conduction. Near the center, the amount ofheat is sufficient to momentarily melt the alloy. The alloy soon coolsand solidifies, but not before material is drawn outwardly away from thespot center to form the center depression as well as the surroundingrim, apparently due to surface tension. The rounded contours aresubstantially and measurably more resistent to wear from contact withthe transducing head.

For controlling the macro texture, i.e. the pattern or arrangement ofspots, it has been found satisfactory to rotate disc 16 at a rate offrom about 10 to about 100 rpm, along with controlling the Q switchingrate of laser 44 for a firing frequency over a range of from about 5kilohertz to about 20 kilohertz. A satisfactory pitch or inter-ringspacing ranges from about 0.25 to 4.0 mils, while a preferred range isfrom 0.5 to 1.0 mils.

A desired result of parameters chosen within these ranges is theenlarged portion of head contact area 32, illustrated in FIG. 6. Thespacing between adjacent spots in the radial direction (pitch), and thecircumferential spacing between adjacent spots, are approximately equalto and can even be less than the average spot diameter of about 1 mil.The result is a substantially uniform, continuous texture comprisedalmost entirely of center depressions and raised rims about thedepressions. The following examples are of approaches within theprescribed texturing parameter ranges.

EXAMPLE 1

An aluminum rigid disc, having a diameter of 8 inches and a thickness of0.075 inches, was plated with a nickel-phosphorous alloy to a thicknessof about 400 microinches. The laser was operated in the fundamental mode(designated TEM₀₀) and a current of 16.5 milliamperes was applied to thelaser, to generate peak power of 0.2 kilowatts. The Q switching rate ofthe laser was maintained at 12 kilohertz, with a pulse duration of about100 nanoseconds, while the disc was rotated at a rate of 25 rpm. Theresult was a circumferential ring in which adjacent laser spots nearlytouched one another, spaced apart by a distance approximately equal tothe average spot diameter. After each revolution of the disc, laser 44was stepped or translated radially of the disc by a pitch of 0.8 mils.200 concentric rings were formed, creating an annular head sector orband with a width, in the radial direction, of 160 mils. Individualspots in the band were observed using a WYKO-3D surface profilometer(phase shifting interferometer), available from WYKO corporation ofTucson, Arizona. The typical and predominant laser spot had a ridgeextended from about 0. 5 to 0. 8 microinches above the nominal surface,and a center depression with a depth of about 1.0 to 2.0 microinchesbelow the nominal surface plane. The average spot diameter was 0.8 mils.

EXAMPLE 2

A nickel-phosphorous alloy was plated onto an aluminum disc as inExample 1, and textured as in Example 1 except that at laser 44 waspowered by a current of 17.5 milliamperes. The resulting spots hadridges from about 1.0 to 2.0 microinches above the nominal surface, withdepressions of about 4.0-5.0 microinches below the nominal surface, withan inter-track pitch of 0.8 mils.

EXAMPLE 3

A nickel phosphorous layer was plated onto an aluminum disc and texturedas in Example 1, except that laser 44 was powered by a current at 17.5milliamperes, and the inter-track pitch was 1.0 mil. The spot structurewas similar to that in Example 2.

EXAMPLE 4

A nickel phosphorous layer was plated onto an aluminum rigid disc andtextured as in Example 1, except that the interring pitch was reduced to0.5 mils and the total width of the area was 150 mils. Adjacent spotstouched one another, with typical spots having ridges raised about 0.6microinches and center depression depths of about 1.2 microinches.

While the generally circular spot configuration is obtained with avertically or perpendicularly oriented laser beam, the laser beam may bedirected onto disc 16 at an inclined angle, e.g. 45 degrees from thehorizontal. The result is an elongation of each spot into an ellipticalor oval shape. When the beam is tilted yet maintained in a verticalplane containing the disc radius, and when the disc rotational speed andfiring frequency are matched to provide a circumferential distancebetween spots approximating the spot diameter, the result is acircumferential ring of adjacent, elongated spots 64 as illustrated on asubstrate 66 in FIG. 7.

It is to be appreciated that the control parameters can be varied toprovide alternatives to the previously discussed patterns, e.g. varyingthe inter-ring pitch so that it increases in the radially outwarddirection, staggering adjacent rings so that spots in each ringcorrespond to regions between spots in its next adjacent rings, randomarrangement of spots, and arrangements in which the laser peak power orspot frequency is progressively decreased for radially outward rings inorder to progressively decrease roughness in the radially outwarddirection in the head contact band.

FIG. 8 shows a substrate 63 with a mechanically textured substratesurface. Depending upon the size of grit selected and the nature of thepaper or cloth supporting the grit, the substrate surface can betextured to a selected roughness to provide a head contact sectordedicated to contact with the transducing head. In sharp contrast withFIGS. 4-7, however, FIG. 8 reveals that the surface of substrate 63 hasan acicular topography characteristic of mechanical texturing. Thetextured surface includes peaks 65 and indentations or valleys 67,irregular in height and depth and characterized by steep slopes andpointed edges or ends.

Furthermore, the pointed edges are areas of stress concentration due tothe cutting action of the grit. Consequently, the tips of the highestpeaks are susceptible to being broken away when contacted by atransducing head moving relative to the substrate. As indicated in thefigure, an upper tip 69 of one of the peaks has been broken away toleave a more planar though not necessarily horizontal upper surface 71.

Once the tips have been broken away from a multiplicity of such peaks,two problems arise, both of which hamper the long term reliability ofthe recording system. First, the multiplicity of generally flat surfaceslike 71 (rather than the original pointed tips) increases the overallarea of surface contact with the transducing head, increasing thestiction and friction problems (commonly referred to as frictionbuild-up). Secondly, the multiple broken away tips tend to adhere to thetransducing head as they break away, build-up in valleys 67 and exposethe magnetic layer in peak regions for possible corrosion sites, orremain free as particulate contaminate, in any case reducing thereliability of the recording system.

The charts in FIGS. 9 and 10 represent comparisons of mechanicallytextured discs with discs textured by a laser in accordance with thepresent invention. More particularly, mechanically textured discs andlaser textured discs were compared both initially and at various stagesof contact start-stop testing. All discs were provided with a 300angstroms thick protective layer of sputtered carbon. In connection withthese figures, it should be noted that actual test results would appearas a series of vertical bars indicating ranges. The line in each Figurerepresent a series of midpoints of such vertical bars.

As seen in FIG. 9, all of the discs initially had a coefficient frictionof about 0.25. In the case of mechanically textured discs, representedby the broken line 68, the coefficient of friction was well above 1.0 by3,000 contact start-stop cycles, and was close to 2.0 after about 10,000cycles. In contrast, the coefficient of friction for the laser textureddiscs remained at about 0.25, as indicated by solid line 70. As thecoefficient of friction is the principal indicator of wear to the headand head contact surface of the disc, the results shown in FIG. 9demonstrate a surprising durability in the head contact area whentextured in accordance with the present invention.

FIG. 10 also illustrates a comparison of mechanically textured and lasertextured discs over numerous contact start-stop cycles, in this casecomparing the envelope of self-excited head gimbal arm vibration. Theself-excited head/gimbal arm vibration energy is defined as the energyspent at the head/disc interface from the time the head overcomes thestiction force to free flying during the start-stop testing. It ismeasured by integrating the strain experienced by the head arm, measuredhere by a capacitance probe, over the aforementioned time period. Thisvalue, measured as the transducing head takes off from the head contactarea when the disc is being accelerated from stop, predicts future wearto the head and head contact region of the disc. As seen from acomparison of broken line 72 representing mechanically textured discsand solid line 74 representing laser textured discs, the mechanicallytextured discs initially exhibited a lower reading, but surpassed thelaser textured discs well before 5,000 contact start-stop cycles, andthereafter remained above the laser textured discs. Again, asubstantially longer useful life is indicated for both heads and disccontact surfaces, when such surfaces are textured to provide smooth,rounded contours in accordance with the present invention.

Further as to both FIGS. 9 and 10, it was found that not only are theaverage values for coefficient of friction and envelope of self-excitedhead gimbal arm vibration improved in the case of laser textured discs,but that ranges of these values remain substantially uniform overnumerous contact start-stop cycles, indicating that laser texturingleads to a more reliable head/disc interface. By contrast, in the caseof mechanically textured discs, the range of values widens as the numberof contact start-stop cycles increases, i.e. the aforementioned verticalbars indicating ranges become longer.

While the preferred embodiments of the invention contemplate a rigiddisc in which the transducing head during reading and recordingoperations is spaced apart from the disc by an air bearing, flexiblemedia can likewise be textured in accordance with the present invention.The principal difference is that a flexible disc or tape remains incontact with the transducing head at all times, and thus substantiallythe entire surface of such disc, rather than a limited head contactband, is textured.

Thus, whether media textured in accordance with the present invention isflexible or rigid, a substantial and surprising increase in durabilityis achieved, measurable principally in its ability to maintain arelatively low coefficient of friction even after numerous contactstart-stop cycles, e.g. 10,000 or more. It is believed that the smooth,rounded contours of the surface discontinuities are a major contributingfactor to increased durability. A further factor is the improved controlof the texturing process, yielding a high degree of uniformity insurface roughness throughout the specially textured surface. The laseris a preferred device for the controlled texturing, but otheralternatives, for example photolithography, plating or etching toachieve smooth and rounded contours, may be employed to texture surfaceareas in accordance with the present invention.

In connection with rigid discs and flying heads, the present inventionaffords the added advantage of providing a surface area dedicated tocontact with the transducing head during accelerations, decelerationsand with the disc at rest. Substantially all of the remaining discsurface area can have a specular finish ideally suited for reading andrecording data.

What is claimed is:
 1. A device for storing magnetically readable data,including:a disc including a substantially rigid, non-magnetizablesubstrate having a substantially planar substrate surface, said discfurther including a magnetizable film deposited over said substratesurface as a layer substantially uniform in thickness, said disc havingan outer surface having a nominal surface plane; and wherein said outersurface includes a plurality of marks, each comprising a depressionextended inwardly of the nominal plane toward said substrate a distancein the range of from 0.5 to ten microinches, and a rounded rimsubstantially surrounding each depression and extended outwardly of thenominal surface plane by a height of at least 0.2 microinches, each ofsaid rims having a diameter in the range of from 0.1 to five mils. 2.The device of claim 1 wherein:said outer surface includes a first areaadapted for surface engagement with a magnetic transducing head, and asecond area adapted for storage of magnetic data, wherein said headduring recording and reading operations is maintained in spaced apartrelation to the second area by an air foil generated by rotation of saiddisc, and wherein said depressions and rims are formed only throughoutsaid first area.
 3. The device of claim 2 wherein:said first area has aroughness of at least 0.5 microinches, while said second area has aroughness of at most 0.1 microinches.
 4. The apparatus of claim 2wherein:said first area is an annular band substantially concentric on arotational axis of said disc, and has a radial dimension exceeding thewidth of said transducing head.
 5. The device of claim 2wherein:adjacent ones of said marks are spaced apart from one another adistance no greater than five mils.
 6. The device of claim 2wherein:said depressions and corresponding rims comprise at least fiftypercent of the surface area of said first area.
 7. The device of claim 2wherein:said substrate includes an aluminum body and a layer of anickel-phosphorous alloy plated on said body, with said depressions andrims formed in said nickel-phosphorous alloy, and wherein saidmagnetizable film is formed over said alloy and replicates the surfacecontours of said substrate surface.
 8. The device of claim 7 furtherincluding:a protective cover layer deposited on said magnetizable filmas a layer substantially uniform in thickness whereby it replicates thesurface contours of said magnetizable film, said outer surface being theexposed surface of said cover layer.
 9. An apparatus for recording andreading magnetic data, including:a rotatable disc, said disc including anon-magnetizable substrate, a magnetizable film deposited over asubstrate surface of said substrate and having a substantially uniformthickness whereby said film substantially replicates the surfacetopography of the substrate, and a cover layer deposited onto themagnetizable film and having a substantially uniform thickness wherebyit substantially replicates the surface topography of the film; and amagnetic transducing head, and means for supporting said head in aselected orientation and for generally radial controlled movementrelative to said disc; said substrate surface having an annular dataregion with a surface roughness of at most 0.2 microinches, and anadjacent annular second region, said substrate surface having a nominalsurface plane and including throughout said second region a plurality ofdepressions extended inwardly of the nominal plane at least 0.25microinches and ridges adjacent the depressions and extended outwardlyof the nominal plane by a height of at least 0.4 microinches, saidridges being rounded and free of sharp edges.
 10. The apparatus of claim9 wherein:said second region lies radially inwardly of the first region.11. The apparatus of claim 10 wherein:said second region has a width,radially of said disc, in the range of from 0.05 to 0.5 inches.
 12. Theapparatus of claim 11 wherein:said depressions and rims are generallycircular, with adjacent ones of said depressions spaced apart form oneanother by at most five mils.
 13. The apparatus of claim 12 wherein:therims have diameters in the range of from 0.1 to 5.0 mils.